The invention relates to an arrangement for the coating of a running material web.
Paper, cardboard etc. are normally produced from natural cellulose fiber. The paper production is a complicated process, which is started by the defibration of natural logs or other plant parts, fibre preparation and finally sheet formation. In the sheet formation one strives to distribute the fibers so evenly as possible. Along with the dewatering, which is carried out by the sheet formation, the fibers are guided more near to each other and are linked, mainly through fluidum bonds.
The paper comprises fibres located in several layers, arbitrarily distributed. If one examines the paper through a magnifying glass, cavities are found between the fibers and the surface is found uneven - everything depending on the micro structure quoted above.
One might have different reasons for the desire to improve this uneven micro structure. One might have, for instance, the desire to reduce the porosity in the paper, in order to be able to apply it as packaging material. Or, it is also important to improve the paper surface itself, in order to be able to print on the paper with a good result. A common method for the surface levelling and the porocity reducing is the application of a layer on the top of the completed paper surface. A normal method is to mix finely divided pigment, for instance clay, to supply this paste on the paper web. The surface smoothness is realized in that one doctors the paste by a trowel blade, a so called scraper knife. The doctored paper is hereafter dried, whereby the supplied paste is attached at the paper web through a certain penetration.
A normal coating method is the guiding of the paper web through a nip located between two drums, in which the paste is supplied by the partial immersion of the other drum in a machine bin filled with paste. The paste excess is doctored off hereafter at a certain distance from the nip and the paste is levelled by a scraper knife. The drum nip can also be replaced by a fountain or a so-called applicator.
Normal web speeds are 600-1000 m/min but even higher speeds, up towards 2000 m/min are applied. The paste excess during the doctoring is hereby often 10-20 ggr. This excess amount is resupplied in to a trough for the return to the machine bin.
A nip method has been developed during the past years, which combines the fountain application and the doctoring in to one member the so-called short dwell technique. A benefit hereby is the short distance between the paste application and the doctoring.
The so far constructed short dwell appliances comprise a defect, which is the minor paste excess amount realized. This is a result of the applied technique, in which the eventual paste excess amount must be compressed out at the rear wall against the running direction of the paper web. This results even in a prolonged penetration extent than was intended for.